The present invention relates generally to an internal combustion engine including a mass airflow based control system and, more particularly, to an improved method and apparatus for controlling an internal combustion engine which is capable of inferring a parameter comprising a ratio of predicted current air charge entering the engine to predicted peak air charge capable of entering the engine.
It is known in the prior art to determine a parameter comprising a ratio of manifold pressure to barometric pressure. Readings of manifold pressure and barometric pressure are employed to determine this parameter. This parameter has the characteristic of going to the value of 1.0 at any altitude and engine speed when the engine maximum airflow is being reached, thus indicating that the driver is demanding maximum torque as opposed to maximum fuel economy. When the ratio approaches the value of 1.0, this indicates to the engine controller that fuel enrichment is required. This parameter, while being advantageous for control systems employing manifold pressure sensors, is not advantageous for mass airflow engine control systems since such systems do not normally employ manifold pressure sensors.
It is also know in the prior art to determine a further parameter, which is essentially equal to the pressure ratio described above. This parameter, which may be called inferred pressure ratio IP, is found by employing the following equation: EQU IP=(AC / (PAC * BP/29.92))
wherein:
AC is the air charge going into the engine measured by a mass airflow meter; PA1 PAC is the peak air charge capable of going into the engine which is inferred from a look-up table; PA1 BP is the barometric pressure surrounding the engine; and PA1 29.92 is a constant equal to standard barometric pressure.
The parameter IP is also used by an engine control system for fuel enrichment determinations. While this parameter may be determined by a mass airflow control system, it is very sensitive to barometric pressure. Thus, if a control system is employed that infers values of barometric pressure, which are sometimes not as accurate as measured values of barometric pressure, the accuracy of the parameter IP will be directly affected by any inaccurate determinations of inferred barometric pressure.
Accordingly, there is a need for an improved mass airflow based control system which is capable of determining a parameter which can be used for, inter alia, enrichment determinations, but which has little sensitivity to inferred barometric pressure.